Problem: I have $19$ pets. $17$ of them are fish and the rest are cats. How many cats do I have?
Solution: We can solve this problem in two ways: $ \begin{aligned}&{17} \text{ fish}\\ +& \underline{{?} \text{ cats}}\\ =&{19} \text{ pets in total}\end{aligned}$ Or: $ \begin{aligned}&{19} \text{ pets in total}\\ -& \underline{{17} \text{ fish}}\\ =&{?} \text{ cats}\end{aligned}$ ${2}$ cats